this derivation will be built:
  /nix/store/rkdhzkinjpcwfgncm73c8b97vf9hys1w-interpolate-conformance.drv
bld4: building rkdhzkinjpcwfgncm73c8b97vf9hys1w-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected